The invention relates to high-speed data transmission in mobile communications systems, especially when a multichannel configuration is used.
In mobile systems, the transmission capacity available at the radio interface is divided among a plurality of users according to a multiple access principle. The most commonly used multiple access schemes include time division multiple access (TDMA), code division multiple access (CDMA) and frequency division multiple access (FDMA). In TDMA systems, communication over a radio path takes place on a time division basis in successive recurrent TDMA frames, each of which comprises several time slots. A short information packet is transmitted in each time slot in the form of a radio-frequency burst of a limited duration, consisting of a number of modulated bits. The time slots are mainly used for conveying control channels and traffic channels. Traffic channels are used for transmitting speech and data, whereas control channels are used for signalling between a base station and mobile stations. An example of a TDMA radio system is the pan-European mobile system GSM (Global System for Mobile Communications).
In a CDMA system, a traffic channel is determined by a unique spreading code assigned to a mobile station, whereas in an FDMA system a traffic channel is determined by a radio channel.
Maximum data transfer rate on a single traffic channel is limited to a rather low level according to the available bandwidth and the channel coding and error coding used in the transmission. For example, in the GSM system the user data rate of a traffic channel employing one time slot was limited to 9.6 kbit/s according to the original specifications, and the radio interface rate was 12 kbit/s. However, this has been found insufficient for many of the new teleservices, such as telefax, video transmission etc., wherefore new mobile systems are being provided with high-speed data transmission services based on so-called multichannel technology. In multichannel technology, a mobile station is provided with a higher bit rate and a greater bandwidth by means of several parallel basic traffic channels (e.g. several time slots). For example in the GSM mobile system, high-speed data service HSCSD (High Speed Circuit Switched Data) is defined in recommendations GSM 01.34, GSM 02.34 and GSM 03.34 of the ETSI (European Telecommunications Standards Institute). In the HSCSD concept, a high-speed data signal is divided into separate data streams, which are then transferred via N subchannels (N traffic channel time slots) at the radio interface and, correspondingly, via N subchannels between the base station and the mobile services switching centre (transcoder). After the data streams have been divided, they are transferred on the subchannels as if they were mutually independent until they are combined at the receiving end. However, logically these N subchannels are parts of the same HSCSD connection, in other words they form one HSCSD traffic channel. The capacity of an HSCSD traffic channel is thus almost eightfold compared to the capacity of a basic traffic channel, which considerably improves the data transfer rate. The GSM HSCSD is capable of supporting a radio interface rate of 96 kbit/s (8xc3x9712 kbit/s) and user rates of up to 64 kbit/s and 76.8 kbit/s (8xc3x979.6 kbit/s) at the radio interface.
The EDGE (Enhanced Data Rates for GSM Evolution) project of the ETSI is in the process of-developing a new modulation method providing a higher data rate per time slot than the present GMSK modulation, while retaining the channel spacing of 200 kHz and the TDMA frame structure. This enables supporting the present HSCSD data services with a lower number of time slots. The new modulation method also makes it possible to provide new data services with a data rate that may be as high as 64 kbit/s per time slot or over 64 kbit/s (n*64 kbit/s) in a multislot constellation. According to the present alternative modulation methods, the radio interface rate is either 28.8 kbit/s or 38.4 kbit/s on a single channel.
A problem related to multichannel technology is how to distribute the data arriving from high-speed data sources into separate channels (subchannels) and how to assemble the data received from the separate channels into the correct (original) order. This problem relates to both the mobile station and the mobile communication network.
In the HSCSD service of the GSM system this has been solved by using subchannel numbering and intra-subchannel frame numbering. During a connection mutually different channel numbers are assigned to parallel traffic channels. High-speed data is divided at the transmitting end into frames, each of which is provided with a channel number which indicates the parallel traffic channel used for the transmission. The frames are divided into parallel traffic channels at the transmitting end in a sequential order following the channel numbering and transmitted to the receiving end. At the receiving end the data contained in the frames is assembled back into a high-speed data signal in the sequential order according to the channel numbers contained in the frames. Furthermore, frame numbering is used within each subchannel to improve the sensitivity of data transmission to relative inter-subchannel transmission delays, which may confuse the order of the frames at the reception. The frame numbering is also transferred in each frame. The arrangement disclosed above is described for example in ETSI/GSM recommendation 03.34 and in Finnish Patent 97187. Such inband channel and frame numbering consumes the channel capacity, which has been reduced by inserting the channel and frame numbering in place of the data in V.110 frames.
The situation changes significantly as a result of the new modulation method of the EDGE. The data rate at the radio interface and the data rate over a leg between a base station and an interworking function (usually located remote from the base station at a mobile services switching centre) are no longer directly compatible or adapted one-to-one, unless entirely new rate adaptation functions are defined between the base station and the interworking function.
The EDGE project suggests several new alternative solutions for this problem. One alternative is to define completely new rate adaptation functions, optimized for the EDGE, between the base station and the interworking function. Another alternative manner is to use existing TRAU formats and physical 16 kbit/s channel structures at an Abis interface. Since data rates exceeding 14.4 kbit/s cannot be rate-adapted into one 14.4 kbit/s TRAU frame, TRAU frames of several Abis transmission channels must be used to provide the higher capacity required by the EDGE radio interface. In this case, the base station must process a higher number of Abis transmission channels than the number of time slots used at the EDGE radio interface. For example, one time slot (channel) at the EDGE radio interface (28.8 kbits) would require two Abis transmission channels with a 14.4 kbit/s TRAU format. Correspondingly, two 28.8 kbit/s EDGE channels (57.6 kbits multichannel configuration) would require four Abis transmission channels.
An advantage of using the present rate adaptation functions is that the EDGE radio interface does not require changes in the Abis interface and the TRAUs. A drawback is the increased complexity of the base station, which results from the base station having to process two different legs with different frames and channel numbers. In the uplink direction the base station must receive the EDGE frames from the radio interface channels, restore the order of data, place the data in the TRAU frames and transmit the frames via a greater number of transmission channels to the interworking function. Correspondingly, in the downlink direction the base station must receive the TRAU frames from the transmission channels, restore the order of data, place the data in the EDGE frames and transmit the frames via a smaller number of channels at the radio interface.
An object of the present invention is to simplify the operation and structure of a base station in a mobile communications system which requires a higher number of transmission channels than traffic channels, and where the traffic and transmission channels employ different frame structures.
The invention relates to methods according to claims 1 and 4, digital mobile communications systems according to claims 7 and 9, a base station according to claim 13 and mobile stations according to claims 16 and 18.
Each high-speed traffic channel at the radio interface requires two or more lower-rate transmission channels between the base station and the interworking function, which is typically located at the mobile services switching centre. Radio frames, such as EDGE frames, are transmitted over a radio interface traffic channel. Transmission channels are used for transmission of transmission frames, such as TRAU frames. Since each transparent call always has at least two parallel transmission channels via which the transmission frames are transmitted, the base station and the interworking function provide the transmission frames to be transmitted with frame and/or channel numbering which indicates the order of the transmitted frames and/or the transmission channel via which the transmission frame was transferred. According to the invention, the base station does not restore the order of the downlink transmission frames according to the aforementioned frame and/or channel numbering, but it places the data of the transmission frames and at least the frame and/or channel numbering in the payload of the downlink radio frame to the transmitted. Instead, the mobile station, which extracts the data and the frame and/or channel numbering of the transmission frames from the downlink radio frames, restores the correct order of the data on the basis of this numbering.
The radio interface also employs two or more traffic channels, for example according to the GSM HSCSD concept. Even in such a case the radio interface does not require separate frame and/or channel numbering to restore the order of the data if the internal timing of the radio system, comprising time slot numbering, is used for this purpose. In other words, it is possible to utilize the natural order of the time slots in the TDM system (e.g. time slot 0 precedes times slot 1, time slot 1 precedes time slot 2, . . . , time slot 6 precedes time slot 7, time slot 7 precedes time slot 0 of the next burst/frame, etc.). If separate numbering is used at the radio interface, the base station can provide the downlink radio frames to be transmitted with some other frame and/or channel numbering. The mobile station restores the order of the received radio frames or the contents thereof on the basis of this other numbering before the data and the frame and/or channel numbering of the transmission frames are separated from the radio frames. The mobile station thereafter restores the order of the data on the basis of the frame and/or channel numbering of the transmission frames. Correspondingly, the mobile station provides the uplink radio frames to be transmitted with the aforementioned other frame and/or channel numbering. The base station does not restore the order of the radio frames on the basis of the other numbering, but it places the data and the frame and/or channel numbering of the radio frames in the uplink transmission frames to be transmitted. The base station also provides the uplink transmission frames to be transmitted with its own frame and/or channel numbering. The interworking function restores the order of the transmission frames or the contents thereof on the basis of this numbering before the data and the frame and/or channel numbering of the radio frames are separated from the transmission frames. The interworking function thereafter restores the order of the data on the basis of the frame and/or channel numbering of the radio frames.
The base station can also insert other control and status information of the transmission frames into the downlink radio frames. The base station preferably inserts the transmission frames into the radio frames as such, so that the base station does not have to identify the frame and channel information at all. Correspondingly, the base station can also place in the uplink transmission frames other control information for the radio frames, preferably the radio frames as such.
Due to the invention some of the functions related to frame and/or channel numbering are no longer carried out at the base station, which leads to a less complicated and more economical base station. The functions are transferred to the mobile station and the interworking function, since the base station forwards the numbering directly from the interworking function to the mobile station or vice versa.
Therefore an objective of the invention is to develop a method and an apparatus implementing the method such that the aforementioned problems can be solved. The objectives of the invention are achieved with a method and a system which are characterized by what is disclosed in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.